@brian.atkins  Adding new Account Ids in PX will create new Accounts instead of updating existing Accounts since the Account Id is a unique identifier. This could cause any account level data that has been captured to be associated with old id only and can not be moved to new id. But all the data associated to user id will be there.

1. Use the API to update the account_id to the new value at the account level (this wouldn’t be possible)
2. Use the API to update the account_id to the new value at the user level (this is one way to update or we can also use PX data loader to do the same). Also updating the account id value in identifier call, so any login users are automatically added to new id.

Here can another solution - Keeping both old and new ids: As removing the old id causes data not associate to new id, we can keep both ids in PX like the account Id can be old account id and creating a custom attribute to store new account id which can be used for reporting.

Hope this helps!
 

You can also check with your CSM team to share more customised approach for the same.